The present invention relates to a coordinate measuring machine having a sensor for measuring an object, and having a housing structure for holding and positioning the sensor.
Further, the present invention relates to a method for collision detection of a coordinate measuring machine having a tactile sensor for measuring an object, and having a housing structure for holding and positioning the tactile sensor.
Furthermore, the present invention relates to a method for correcting image data of a sensor, measuring in a contactless fashion, in particular an optical sensor, of a coordinate measuring machine.
Three-dimensional coordinate measuring machines having up to six degrees of freedom are used in wide areas of industrial metrology, in order to measure objects under the highest accuracy requirements. For example, a shape of workpieces produced by machine is checked in this way for quality control. To this end, coordinate measuring machines are known in principle that operate, for example, with optical sensors, or else with tactile sensors.
As a rule, the optical sensors are optical probe heads such as, for example, triangulation sensors or video cameras that have an electronic evaluation unit. In the example of a video camera, a system clock of this electronic evaluation unit is fixed by the video frequency of the camera. Such optical sensors can be advantageous when the aim is to measure an object in a contactless fashion. However, optical sensors encounter limits when objects of complex shapes are to be measured, since what can then happen is that details of the object do not lie in an image recording area of the optical sensor, and therefore cannot be detected.
Such optical sensors are, for example, sold by the applicant under the designation of “ViSCAN®”.
A tactile sensor is provided, as a rule, in the form of a probe tip that is mounted with the ability to be deflected in three dimensions, and whose deflection can be detected by means of the tactile sensor. For the measurement operation, the probe tip or the feeler is moved so far up to the object to be measured that it touches a desired measurement point on the object. The probe tip and the sensors detecting the deflection of the probe tip are also denoted together as probe head. As a rule, the probe head is fitted in a suitable displacement device that enables a three-dimensional movement of the probe head with up to six degrees of freedom. The spatial coordinate of a probed measurement point can be determined with high accuracy from the respective position of the probe head and the relative position of the feeler pin relative to the probe head, which is detected with the aid of the sensor.
Tactile sensor are described, for example, in document DE 10 2004 011 728 A1, and are, for example, sold by the applicant under the designation of “VAST® XXT”.
In order to be able to fulfill the high demands for accuracy placed on the measurement results in the case of such tactile or optical sensors, it is necessary to be able to accurately detect a movement of the sensor during a measurement operation. With tactile sensors, movements of the sensor during touching or probing can lead to distortions that necessarily must be taken into account in an evaluation of the measurement results. For optical sensors, for example for CCD cameras, a movement during the measurement operation leads to a “smearing” of the recorded image which then appears unsharp. Although it is possible to eliminate from the measurement result the movement of a sensor during the measurement operation by subsequent computational after processing, it is at the expense of absolutely requiring adequately exact knowledge of the movement operation and/or the path data of the sensor during measurement.
In order to displace the tactile and the optical sensors, a housing structure that enables a displacement of the sensors in all three spatial directions (X, Y, Z) is routinely provided. Furthermore, it is possible to provide a so-called rotating/pivoting joint (DSG) that enables the sensor to be aligned along a desired direction vector. By way of example, such a DSG is sold by the applicant under the designations of “DSE” and “RDS”.
During the operation of a coordinate measuring machine, the position of the sensor or of the sensor head is typically determined along the three spatial directions and stored in the form of position data. Use is made to this end of linear or rotational measurement systems which determine the position and orientation, respectively, with a readout clock typical of the coordinate measuring machine. This readout clock is typically in a frequency range of from a few tens of hertz (Hz) up to a few kilohertz (kHz).
The position data are used for the purpose of creating a position profile and, by differentiating this variable, of calculating the speed, or, by renewed differentiation, of calculating the acceleration of the sensor or of the housing structure. In this case, however, the speed or the acceleration of the sensor or of the housing structure is not acquired directly. Owing to the fact that the temporal variation in the position of the sensor requires the measurement of a plurality of positions which are present only in a limited number, and thus density, because of the measurement frequency of the readout rate, the variation and/or differentiation also can be determined only with limited resolution. As is generally known on the basis of the Nyquist theorem, there is, however, a need for a minimum resolution in order to be able to detect dynamic changes and/or wave movements. Because of the limited resolution, discontinuous changes such as arise in the event of mechanical probing and/or collision also can only be detected with difficulty at present or must have a certain minimum size in order to be detected.
Because of structural boundary conditions, the Abbe's comparator principle is, in addition, violated in the case of most position determining systems of coordinate measuring machines during construction. Consequently, a distortion of the system can give rise to tilt errors during measurement, and dynamic effects can be taken into account only conditionally. Although these effects are at present approximately described via model parameters and taken into account as correction values during measurement, knowledge of the actual circumstances is desirable for an accurate measurement, since a dynamic distortion of the coordinate measuring machine can certainly amount to several μm, however.
It would then be possible to monitor more effectively the probing behavior of a tactile sensor or the path data of a sensor that measures in a contactless fashion, and to correct more effectively the measurement results of both tactile and, in particular, optical sensors in order thus to obtain more accurate measurement results.
An objective object of the present invention therefore firstly consists in more effectively determining a movement behavior of a sensor of a coordinate measuring machine of the type mentioned at the beginning.
A further objective object consists in using the movement behavior thus known to more effectively monitor the touching of a tactile sensor.
Yet a further object of the present invention therefore consists in using the movement behavior thus known to more effectively correct the measurement results of a sensor that measures in a contactless fashion, in particular an optical sensor.